Poseidon SE7EN User Manual
Appendix 3 Page 115
Tips on breathing
Breathing underwater on a closed-circuit rebreather, such as the Poseidon SE7EN, is
somewhat different from breathing on land, or breathing with conventional scuba gear. As the
diver exhales, the counterlungs both expand. As the diver inhales. the counterlungs contract.
The direction of gas flow through the breathing loop is governed by the two check-valves in
the bottom portion of the mouthpiece. The incorporation of two separate, over-the-shoulder
counterlungs on the SE7EN helps to minimize the effort required to breathe underwater, but
there are a few tips that make breathing easier.
The most important thing is to maintain an optimum volume of gas in the breathing loop. If
there is too much back-pressure when exhaling (often felt in the cheeks), or if the overpressure
relief valve on the exhale (left) counterlung “burbs” gas at the end of an exhaled breath, then
the loop has too much gas, and some should be vented (e.g., by exhaling through the nose).
If the counterlungs “bottom out” and/or the Automatic Diluent Valve (ADV) in the mouthpiece
is triggered on a full inhalation, then there is not enough gas in the breathing loop. This condition
should be corrected automatically by the ADV.
Tips on buoyancy control
Controlling buoyancy while diving with a rebreather is considerably different from buoyancy
control with conventional open-circuit scuba. To begin with, whereas a scuba diver needs to
manage buoyancy characteristics of two separate factors: the Buoyancy Control Device
(BCD), and the exposure suit (i.e., a wetsuit or a dry suit). A rebreather diver must manage
both of these, as well as the breathing loop of the rebreather. A complete discussion of
buoyancy control with closed-circuit rebreathers is beyond the scope of this Manual. However,
the following tips might be useful.
Although most divers probably do not realize it, fine trim for diving with conventional scuba
gear is achieved through breathing. On each inhalation, the diver’s lungs expand and buoyancy
is increased. The opposite occurs on exhalation. However, this does not occur with a rebreather
(the Poseidon SE7EN included), because the buoyancy increase caused by expanding the
lungs on an inhaled breath is offset by the decreasing volume of the counterlungs (and vice
versa). This may at first be disconcerting for an experienced scuba diver trying a rebreather for
the first time, because an inhalation done subconsciously to slightly increase buoyancy has no
effect. However, with practice, it becomes advantageous to be able to hover in the water with
perfect buoyancy, while breathing continuously.
The quickest and easiest way to fine-tune buoyancy with a rebreather is via addition and
removal of gas to or from the breathing loop. To increase buoyancy slightly, a small amount of
gas can be added to the breathing loop via the ADV (either by manually engaging the purge
button, or by making an especially deep inhaled breath). For minor adjustments in buoyancy,
it is usually easier to control it with your breath rather than the purge-button since it could
easally give a bit too much gas. To decrease buoyancy slightly, one need only exhale through
the nose to vent gas out of the breathing loop (except when certain kinds of full-face masks
are used).
New rebreather divers often have the most difficulty in very shallow water, where a slight
change in depth yields a proportionally large change in displacement (and, hence, buoyancy).
This is especially true when the diver begins to ascend, which causes the counterlungs to
expand, leading to increased buoyancy, leading to further ascents, and expanding loop
volume. This can lead to a “run-away” ascent that can be difficult to control. For this reason,
it’s useful practice for rebreather divers to be in the habit of venting gas through the nose
whenever ascending; particularly from very shallow depths.
Venting water from the loop
Even if a diver is very careful to prevent water from entering the breathing loop, there will always
be some water collecting due to condensation. Most of this will form on the “exhalation” side
of the breathing loop, between the mouthpiece and the CO
2
absorbent cartridge, and will
generally collect in the exhalation (right-hand) counter lung. Sometimes, water will collect in the
exhalation hose, immediately downstream of the mouthpiece. If this water is sufficient to cause
gurgling noises with each breath, it can be poured into the exhalation counter lung by looking
upward and holding the hose in such a way so as to dump the water towards the right-hand
shoulder port. In most cases, the water that collects inside the exhalation counter lung will not
disrupt the function of the Poseidon SE7EN in any way, so it can be safely ignored. However,
sufficient quantities of water could be returned to the breathing loop if the diver becomes
inverted, so it may be desirable to vent this water from the breathing loop altogether.
To do this, the diver should first become negatively buoyant, or attach to a secure object on
the bottom. The breathing loop volume should be increased to at least 75% of maximum
capacity by manually adding diluent via the ADV. The loop vent valve at the bottom of the
exhalation counter lung should be rotated counterclockwise maximally to minimize the cracking
pressure. While in an upright orientation, the diver should then compress both counter lungs
by squeezing them against the chest with the elbows and upper arms, while simultaneously
exhaling through the mouth and depressing the loop vent valve to open it. If done correctly,
water will be expelled from the loop vent valve first, followed by a stream of gas bubbles. After
the water has been flushed, the loop vent valve can be tightened by rotating clockwise, and
the breathing loop volume and PO
2
can be restored to normal.
A small amount of condensation may also collect in the inhalation portion of the breathing loop,
between the CO
2
absorbent cartridge and the mouthpiece. Normally, this will only be a small
volume of water, and most will be absorbed by water trap.
